U.S. Pat. No. 5,021,711 has already disclosed a sealing foil and associated lamp having this foil. To provide better protection against oxidation, the foil is provided with a protective layer of Al, Cr, Si, Ti or Ta. The thickness is 5 to 100 nm.
A similar technique is known from CA -A 1135781, in which layers of Ta, Nb, V, Cr, Ti, Y, La, Hf or Sc are used for the same purpose. The layer thickness is 10 to 200 nm.
In practice, partial chromium-plating is generally used to protect the molybdenum foils from oxidation in the region of the foil-pin welded joint. In this very laborious procedure, the welded joints produced between pin and foil by resistance welding are manually placed into a sand-like medium up to the height to which the chromium plating is to take place. In an environmentally polluting process, the partial deposition of chromium is carried out by chemical reactions. This chromium deposition (oxidation protection) results in an increased ability of the foil-pin joints to withstand high temperatures. A thermal load of up to approx. 550° C. is then possible.
For some lamps, it is not the oxidation of the foil-pin joints which is responsible for the failure of the foil seal, but rather the attack of the corrosive fill constituents (for example metal halides) or filling gases on the molybdenum foil. To limit this attack, the molybdenum foil has hitherto been sand-blasted, which leads to an improvement in the glass-metal joint. However, the sand-blasting leads to high levels of scrap during resistance welding, since it causes nonconductive Al2O3 particles to remain on the surface of the Mo foil. Moreover, the wear to the resistance welding electrodes is increased very considerably. In the case of sand-blasted foils, it is necessary to replace the electrode after only approx. 70 welds (compared to a replacement interval of approx. 1000 welds for an untreated foil), and consequently a frequent change of the electrodes is required.
DE-A 199 61 551 which corresponds to U.S. Ser. No. 09/705,026 has disclosed the use of Ru containing foils in lampmaking. This document recommends uniform coating of at least one side of the foil.